It often becomes desirable to locate (find) an object that has been previously buried. Of particular interest in this regard are portions of buried utilities such as telephone, electric power, water, gas and the like. For example, in the case of buried telephone cables it is routinely necessary to obtain re-access to such buried objects as splice points, cable stubs, pull boxes, repair sleeves, branches or laterals, etc.. Re-access presupposes location or finding of the buried object; and the degree of effort (expense) required for re-access is directly related to the effectiveness and accuracy of the apparatus and system utilized for the locating task.
Apparatus and systems for locating buried objects in accordance with the prior art of which we are aware have been deficient in both effectiveness and accuracy. Known prior art includes use of a metal ring as a passive marker element and a metal detector as the active apparatus. Such apparatus is unable to distinguish the metal ring from any other metal object with the same field disturbance. Known prior art also includes use of magnets as the passive marker element and a magnetometer as the active apparatus. Magnets which are strong enough to be detectable without false reading due to changes in the earth's magnetic field are expensive and have a relatively short life (2-5 years). Neither of these prior art arrangements is capable of producing a reliable positive indication of only the object in question (passive marker element). In addition, neither of these prior art arrangements is capable of pin-pointing the location of the passive marker element with an optimum degree of accuracy.
In our co-pending continuation application Ser. No. 523,263, filed Nov. 13, 1974, there is disclosed a system utilizing a buried marker device in the form of a passive element which is an inductor-capacitor combination tuned to a predetermined frequency. The marker device is energized by pulses of electromagnetic wave energy from a transmitter having the predetermined frequency and then itself transmits energy to a receiver during the intervals between the pulses. The transmitter-receiver is carried by an operator and contains sensory indicia responsive to received energy and the location of a receiver antenna loop relative to the passive element.
In this application, we disclose systems and methods for locating buried markers which approach the results of the invention of our co-pending application as to sensitivity, accuracy and effectiveness, but which are particularly advantageous from the standpoint of simplicity and economy, as will be apparent from the following description taken in accordance with accompanying drawings, forming a part of this application.